1. Field of the Invention
The present invention generally relates to lens arrays. More particularly, the present invention relates to multiple row lens arrays aligned with multiple row linear image bars.
2. Description of the Related Art
Many presently available imaging systems, such as printers, copiers, and scanners, employ gradient index lens arrays to image light. One manufacturer, Nippon Sheet Glass Company, Ltd., produces these lens arrays under the trade name "SELFOC," a registered mark in Japan. These lens arrays are generally discussed in an article entitled "Office applications of gradient-index optics," by James D. Rees, published in SPIE, Vol. 935, Gradient-Index Optics and Miniature Optics, in 1988, the contents of which are hereby incorporated by reference.
A gradient index lens array usually comprises an array of light conducting and imaging fibers or rods possessing a refractive index distribution across their cross-sections that varies parabolically in an outward direction from their center axes. Typically, these fibers are cylindrical.
FIG. 1 shows an example of a conventional two-row gradient index lens array 100. As shown in FIG. 1, optical fibers are arranged side-by-side in two parallel rows in a direction orthogonal to their mechanical axes, which are preferably also their optical axes. Also, the fibers in the lens array 100 are "close packed"; that is, the fibers are positioned with little or no space therebetween. For example, the rows are arranged in an interlocking fashion (e.g., staggered with respect to each other with little or no space between the rows), so that the fibers in one row "fit" in gaps created by adjacent fibers in the other row.
The lens arrays are typically used with image bars, such as LED or liquid crystal arrays, to form images. FIG. 2 shows an example of a conventional LED printer imaging system employing a two-row lens array. Here, an LED (light emitting diode) array 200 made up of a single row of LEDs is arranged above a two-row lens array 210. LEDs in LED array 200 are activated in accordance with image data from, for example, a digital signal source. Light from the LED array 200 enters the top of the lens array 210. Lens array 210 images the light onto a photoreceptor 220.
Due to the irradiance profile that is characteristic of gradient index lenses, it is essential that lens arrays and LED arrays are properly aligned with respect to each other; that is, that the LED row is symmetrically arranged with respect to the lens rows. Otherwise, images will contain undesirable "banding." Typically, banding appears in images as extraneous lines positioned at a spatial frequency, such as one cycle/mm, which is discernible to the human eye. Alignment of such lens arrays and LED arrays is discussed in an article entitled "Method to Laterally Align a SELFOC Lens Array in an LED Printbar," by James D. Rees, Gradient-Index Optical Systems, 1991 Technical Digest Series Volume 9, Apr. 8-9, 1991, the contents of which are hereby incorporated by reference.
To better understand the positioning necessary for proper alignment, FIGS. 3A and 3B illustrate exemplary end views of the two-row lens array 210 and one-row LED array 200, viewed from perspective A--A as shown in FIG. 2.
FIG. 3A shows an end view of lens array 210 properly aligned with LED array 200. As shown in FIG. 3A, LED array 200 is symmetrically arranged with respect to lens array 210; that is, LED array 200 is positioned halfway between center axes of the lenses in one row and center axes of the lenses in the other row. In this way, the irradiance of the light output from one lens row matches the irradiance output from the other lens row. When combined, the light from these lens rows forms an image without a disproportionate amount of irradiance from either lens row.
If LED array 200 is asymmetrically arranged with respect to lens array 210, arrays 200 and 210 are misaligned. For example, as shown in FIG. 3B, LED array 200 is positioned directly above a center axes of the right lens row. Under this arrangement, the LED row is biased toward the right lens row. This results in non-uniformity contributions from each lens row, generating irradiance modulation at a spatial frequency of approximately one cycle/mm, which is noticeable as undesirable bands to the human eye. To compound this problem, this arrangement results in a high amplitude of irradiance modulation, causing the bands to be even more noticeable. Thus, it is desirable to maintain proper alignment of the lens array with the LED array.
In contrast to the one-row LED array 200 shown in FIGS. 2-3, multiple-row LED arrays can enhance resolution in imaging systems. For example, a two-row LED array can comprise two rows of LEDs, each comprising a 21 .mu.m by 21 .mu.m square. Within each row, the LEDs are spaced at 42 .mu.m centers (e.g., the centers of adjacent LEDs are 42 .mu.m apart).
Also, the LED rows are positioned so that the LEDs of one row are staggered with respect to the LEDs of the other row. With appropriate buffering, the images formed by the LED rows can be combined to form scan lines having a resolution of 1200 spi (spots-per-inch).
However, conventional two-row lens arrays, like the one shown in prior art FIG. 1, cannot be properly aligned with multiple-row LED arrays to avoid banding in images. Such an LED array requires that both lens rows be symmetrical with respect to the first LED row as well as the second LED row. Such an arrangement, however, is not possible with a conventional two-row lens array. Thus, it is desirable to provide a lens array that can be properly aligned with a multiple-row LED array such that banding is avoided in images produced by the lens array and LED array.